Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income Available to Common Shareholders	$ 54,310	$ 52,128	$ 54,097
Available-for-Sale Securities
Unrealized (loss) gain on available-for-sale securities arising during period	7,396	(1,685)	1,713
Reclassification adjustment for net gains	(4,476)	(3,513)	(2,130)
Other comprehensive (loss) income related to available-for-sale securities	2,920	(5,198)	(417)
Defined Benefit Pension Plan
Amortization of actuarial net gains	849	847	975
Recognition of unrealized losses	(11)	(690)	(40)
Change in underfunded pension liability	838	157	935
Other comprehensive (loss) income before income taxes	3,758	(5,041)	518
Tax effect	(1,391)	1,861	(191)
Other comprehensive (loss) income, net of tax	2,367	(3,180)	327
Comprehensive income, net of tax	$ 56,677	$ 48,948	$ 54,424